The present invention relates generally to optical amplifiers, and more particularly to an optical amplifier arrangement having a controller with a user-adjustable slew-rate limiter.
Currently, transmission systems employed in the cable television industry provide two-way transmission of information (e.g., video, multimedia and/or data) between the headend and a plurality of subscribers. Typically, the headend transmits the information destined for individual subscribers in an optical format, via one or more fiber optic links to one or more optical nodes. Each node converts the optically formatted downstream information into electrical signals for distribution, typically via a cable plant having a hybrid fiber/coax (HFC) architecture, to individual subscribers. In addition to receiving the downstream information, each individual subscriber may generate information in the form of voice, data, or a combination thereof, destined for the headend. En route to other subscribers or service providers, the subscriber-generated information is segmented by the coaxial cable plant and passes it to the node for conversion into an optical format for transmission to the headend.
Such transmission systems typically employ optical amplifiers along the fiber optic links to amplify the optical signals being transmitted. One example of a conventional optical amplifier is a rare-earth doped optical amplifier, which uses rare-earth ions as the active element. The ions are doped in the fiber core and pumped optically to provide gain. The silica fiber core serves as the host medium for the ions. While many different rare-earth ions such as neodymium, praseodymium, ytterbium etc. can be used to provide gain in different portions of the spectrum, erbium-doped fiber amplifiers (EDFAs) have proven to be particularly attractive because they are operable in the spectral region where optical loss in the fiber is minimal. Also, the erbium-doped fiber amplifier is particularly useful because of its ability to amplify multiple wavelength channels without crosstalk penalty, even when operating deep in gain compression. EDFAs are also attractive because they are fiber devices and thus can be easily connected to other fiber links with low loss.
Optical amplifiers often employ electronic feedback arrangements to control the output power from the amplifier. For example, the feedback arrangement may be used to provide a constant gain or a constant output power. One limitation of conventional optical amplifiers that employ a feedback arrangement to control the output power is that they typically offer a fixed frequency response. This presents a problem if the modulation frequency of the input signal is in resonance with the frequency of the feedback control loop. In this case the output power from the amplifier may undergo an additional amplification on the output that is undesirable. To avoid this problem, manufacturers typically set the frequency response to a fixed value that is low enough so that most frequencies at which the input signal is likely to undergo modulation will not create a resonance condition. While this is often a satisfactory approach, it hampers the response time of the optical amplifier.
Accordingly, there is need for a more flexible optical amplifier arrangement whose frequency response can be controlled to offer a fast response time and the ability to properly handle low frequency modulated input signals under appropriate circumstances.
In accordance with the present invention, a method and apparatus is provided for controlling the optical output power from an optical amplifier arrangement. The arrangement includes a rare-earth doped fiber for imparting gain to an optical input signal propagating therethrough, a pump source for supplying pump power to the rare-earth doped fiber, and a tap for receiving a portion of the output power generated by the rare-earth doped fiber and converting it to a control signal. A controller is also provided for receiving the control signal and generating a bias current in response thereto for driving the pump source. The method begins by receiving an optical input signal that is being amplitude modulated at a prescribed frequency. The slew rate of the controller is adjusted so that the bias current drives the pump source to generate pump power that cannot vary at a rate greater than a slew-rate limit established by the controller. In this way resonance between the input signal and the frequency of the feedback control loop can be avoided.
In accordance with one aspect of the invention, the optical output power from the optical amplifier is monitored and the slew rate is automatically adjusted in accordance therewith.
In accordance with another aspect of the invention, the power of the optical input signal is monitored and the slew rate is automatically adjusted in accordance therewith. In some cases the slew rate is automatically lowered when the power of the input signal fluctuates at a rate greater than a prescribed value. Moreover, the slew rate may be automatically increased to its previous value when the power of the input signal no longer fluctuates at a rate greater than the prescribed value.
In accordance with another aspect of the invention, an optical amplifier arrangement is provided which includes a rare-earth doped fiber for imparting gain to an optical input signal propagating therethrough. A pump source supplies pump power to the rare-earth doped fiber. The arrangement also includes a tap for receiving a portion of the output power generated by the rare-earth doped fiber and converting it to a control signal. A controller receives the control signal and generating a bias current in response thereto for driving the pump source. The controller includes a user-adjustable slew rate limiter for selectively adjusting the slew rate of the controller.
In accordance with yet another aspect of the invention, the user-adjustable slew rate limiter is hardware-controllable. Alternatively, the user-adjustable slew rate limiter may be software-controllable.